It is known that a composite tape comprising a thermoplastic electro-insulating polymeric matrix and a plurality of metal wires, which are capable of generating heat when an electric current is applied thereto and which are embedded within the matrix, is useful as a heating element. This type of heating element is useful for forming a large area of heat source and, therefore, is beneficially built in a heating device having a large surface area, such as a heating board or a heating carpet.
In a conventional process for producing a composite tape having heat-generating metal wires embedded therewithin, a thermoplastic electro-insulating polymer is melted in an extruder and is extruded through a crosshead die of the extruder to form a tape-shaped stream of the melted polymer, while a plurality of the metal wires are fed through the crosshead die into the tape-shaped stream of the melted polymer to provide a composite stream, and the composite stream is cooled to form a solidified composite tape.
The above-mentioned conventional process has the following disadvantages:
(1) Since the metal wires are incorporated into the polymer matrix while the matrix is in the fluid melt state, the thickness of the polymer layer surrounding each metal wire frequently becomes uneven. Therefore, it is very difficult to locate the wires exactly in the middle portion of the polymer matrix layer.
(2) The arrangement of the metal wires in the polymer matrix frequently becomes uneven because the wire can easily move while the polymer matrix is in the fluid melt state.
(3) In the extruding procedure, only a thermoplastic polymer which is not crosslinked can be used. A crosslinked thermoplastic polymer cannot be melted and, therefore, cannot be used for the extruding procedure. Accordingly, after the metal wires are embedded, the polymer matrix should be crosslinked so as to enhance the heat resistance thereof.
Usually, the crosslinking procedure is carried out by applying electron beams to the polymer matrix under a high voltage. This procedure is extremely dangerous for workers due to the high electroconductivity of the metal wires. Also, the crosslinking effect cannot be attained at portions of the polymer matrix which are obstructed by the metal wires from the electron beams. And since the non-cross-linked portions of the polymer matrix exhibit a poor heat resistance, the resultant products are sometimes useless.
In another conventional process for producing a composite tape containing metal wires, a pair of primary thermoplastic polymer tapes are fuse-bonded side by side while a plurality of metal wires are interposed between the primary thermoplastic polymer tapes. This conventional process has the following disadvantages:
(1) When the primary tapes are fuse-bonded to each other, air bubbles are formed around the metal wires interposed between the primary tapes. When the resultant composite tape is used as a heating element, the air bubbles are heated and expanded, and the expanded air bubbles sometimes cause the bonded primary tapes to be separated from each other or to be ruptured.
(2) It is very difficult to fuse-bond primary tapes having a very small thickness of 200 microns or less without breaking the primary tapes.
(3) The crosslinking procedure for the resultant composite tape involves the same difficulty as that mentioned above.